There is a bad rattling noise coming from the back off my motor (a 25hp Kohler). I was guessing it is the u joint but i don't know. It will stop in the higher rpms. Anyhelp would go I just hope its not the motor.

Mine does the same thing. I'm pretty sure the key between the shaft and the yolk has a little slop in it. Allows it to clatter at low rpm's but clears up when you rev the engine. I also thought it was the u joints replaced them and it still did it. Mud buddy told me to put a heavy tack weld on the key way to the shaft and that stopped alot of the clattering! Or replace the shaft!

There are bronze bushings in the shaft tube that the shaft rides in. If these get worn the shaft will actually whip in the shaft tube making some pretty bad noise. Dont know if thats ya problem but just something to think about.

What's the condition of the prop?
Could be harmonics....

The prop looks good and it is full on new grease. I guessed if the prop was messed up the higher the rpm the worse it would get.

Have someone wiggle the prop back and forth and watch the key way in the drive shaft and you will see the slop if that is what it is!

got a buddy who has the same problem. he replaced the shaft about 5 or 6 months ago and it recently started squealing. it sounds like the belt squeaking on a car or lawnmower but as we all know there aint no belts on a longtail. he tried callin mudbuddy but they wouldnt tell him anything.

I am thinking either bushings or drive shaft. I had one of the first go devil's made many years ago and it started squealing after jumping a beaver dam. Turned out my drive shaft was bent and I had to replace it. The noise it was making was like scratching your nails across a blackboard.